1. Field of the Invention
The present invention relates to an image reading method and apparatus for obtaining an image signal representing a radiation image from a stimulable phosphor sheet on which the radiation image has been recorded, by exposing the sheet to stimulating rays.
2. Description of the Related Art
Image recording and reproduction by reading a recorded radiation image to obtain an image signal representing the image and then applying appropriate image processing to the image signal has been carried out in various fields. For example, a system which can obtain reproduced images having preferable quality such as preferable contrast, sharpness, and graininess has been developed (see Japanese Patent Publication No. 61(1986)-5193). In this system, an X-ray image is recorded on a film having a low gamma value designed to be suitable for subsequent image processing, and the film recording the X-ray image is read to obtain an electrical signal. A visible image such as a copied photograph is reproduced after image processing has been carried out on the electrical signal (image signal).
The applicant has already proposed a radiation image recording reproducing system using stimulable phosphor which emits light, upon exposure to stimulating rays such as visible light, in accordance with stored radiation energy originated from radiation (such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, electron rays, and ultraviolet rays) having been irradiated thereon (see Japanese Unexamined Patent Publication Nos. 55(1980)-12429, 56(1981)-11395, 55(1980)-163472, 56(1981)-164645, and 55(1980)-116340, for example). In this system, a radiation image of a subject such as a human body is recorded on a stimulable phosphor sheet and the sheet is scanned with stimulating rays such as a laser beam to cause the sheet to emit light. The light is photoelectrically detected by reading means such as a photomultiplier to obtain an image signal. Based on the image signal, the radiation image of the subject is then output as a visible image on a recording material such as a photosensitive material or on a CRT display.
As means in such a system for photoelectrically reading light emitted from a stimulable phosphor sheet, an image reading apparatus which comprises a line sensor having a plurality of solid state photoelectric conversion devices placed in line has been proposed (Japanese Unexamined Patent Publication Nos. 60(1985)-111568 and 60(1985)-236354, for example). The image reading apparatus such as those described therein irradiates a streak of stimulating rays on a stimulable phosphor sheet by using a light source array comprising LED's or semiconductor lasers placed linearly in order to cause the LED's or the semiconductor lasers to emit light simultaneously, or by using a combination of a non-directional light source such as a fluorescent lamp or a Xe lamp and an aperture such a slit or a row of small holes. The light emitted from a portion whereon the stimulating rays have been irradiated is photoelectrically read by the line sensor.
By using a line sensor as has been described above, a light source and reading means can be made small, which leads to down-sizing of the apparatus. Furthermore, since the line sensor can be placed close to a stimulable phosphor sheet, light detection efficiency can be improved. Moreover, since a line sensor can carry out reading of one line at a time, a radiation image can be read quickly.
However, in a reading apparatus using a line sensor, since stimulating rays are irradiated on a stimulable phosphor sheet by using a slit or the like, the amount of stimulating rays to be actually irradiated on the sheet is reduced by the slit even when the amount of rays from the light source is sufficient. Therefore, sufficient-amount stimulating rays cannot be irradiated on the stimulable phosphor sheet and the light emitted from the sheet consequently decreases. As a result, the S/N ratio of an image signal obtained by the line sensor deteriorates. When a light source array such as LED's is used, a sufficient amount of stimulating rays can be obtained. However, since a variance in intensity of the rays is observed between near the center of each LED and at each space between LED's, the light emitted from a sheet has a similar intensity variance. As a result, when an image signal obtained thereby is reproduced, an image having a linear density variance can be generated. Furthermore, some light source arrays have a slit to cause a beam emitted therefrom to be thin in order to enhance sharpness of a reproduced image, which leads to S/N ratio deterioration similar to the one described above, due to reduction in the amount of stimulating rays caused by the slit.